This invention is in the field of vibrating beam accelerometers and the method of manufacturing the same, and in particular to the metal employed in fabricating the surface electrodes on the vibrating beams.
Oscillators, such as clock crystal oscillators, surface acoustic wave sensors, and various vibrating beam transducers are typically formed of such materials as silicon, quartz, and zirconia with surface electrodes formed of metal. The electrode material is usually gold, which is a highly stable and electrically conductive noble metal. Under benign circumstances, the gold electrodes function effectively. However, the radiation capture cross-section of gold to gamma radiation is relatively high. When such devices having gold electrodes are exposed to radiation environments, gamma capture due to the high radiation capture cross-section results in heating of the electrode material. The heating causes strain between the thermally mis-matched materials, which in turn causes bias errors. Over time the bias errors build up, thereby reducing the quality of the information output by the device.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. The resonators is often formed of tuned beams of a material, such as silicon, quartz, or zirconia, with metal electrodes deposited on a surface thereof. The surface electrodes are usually formed of gold. In one such mechanical resonator, the beams are coupled between an instrument frame and a proof mass suspended by a flexure. usually formed of gold. In one such mechanical resonator, the beams are coupled between an instrument frame and a proof mass suspended by a flexure.
In operation, a drive voltage is applied to the surface electrodes to cause the beams to vibrate transversely at a resonant frequency. The vibration frequency is monitored in the same or other surface electrodes. The beam vibration frequency changes as the result of tensile and compressive forces applied to the accelerometer by changes in external acceleration. The acceleration force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams. Such vibrating beam accelerometers are more fully described in each of U.S. Pat. No. 5,334,901, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,110, entitled DUAL PENDULUM VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,111, entitled CAPACITIVE DRIVE VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,948,981, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,996,411 entitled VIBRATING BEAM ACCELEROMETER AND METHOD FOR MANUFACTURING THE SAME; and U.S. Pat. No. 6,119,520, entitled METHOD FOR MANUFACTURING A VIBRATING BEAM ACCELEROMETER, the complete disclosures of which are incorporated herein by reference. Vibratory force transducers have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, entitled METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER and 4,945,765 entitled SILICON MICROMACHINED ACCELEROMETER, the complete disclosures of which are incorporated herein by reference.
The present invention provides minimization of the time that bias errors persist, thereby reducing the build up of position and velocity errors of inertial navigation systems. Minimization of the time that the bias error persists is accomplished by the present invention by reducing the thermal time constant of the error driver, which is the thermal gradients in the crystal resonator. Minimization of the thermal gradients in the crystal resonator is provided by utilization of an electrode material that is formed of a material having properties as compared to gold of a low radiation capture cross-section and a high electrical conductivity. Preferably, the electrode material is a metal. The low radiation capture cross-section is a radiation capture cross-section to gamma radiation that is low as compared with gold. The radiation capture cross-section to gamma radiation as compared to gold is less than 0.75, and preferably less than about 0.55. The electrical conductivity of the material is comparable to gold. For example, the electrical conductivity of a preferred material is greater than about 5xc3x97106 Ohm-meters.
According to one aspect of the invention, the electrode material is a material, preferably a metal, selected from a group of materials that exhibit the desired properties. For example, the preferred material is one selected from the group consisting of aluminum, chrome, molybdenum, and other equivalent materials that exhibit the desired properties. Additionally, the preferred materials exhibit a low thermal coefficient of expansion and are depositable on the transducer beam or beams in thin films according to conventional methods well-known in the art.
The present invention further provides a transducer for use in any of an accelerometer, a piezo-resistive strain gauge, a piezoelectric transducer, and a surface acoustic wave transducer, the transducer having a beam and an electrode deposited on the beam, wherein the electrode is formed of a material having properties as compared to gold of: a low radiation capture cross-section, and high electrical conductivity.
According to other aspects of the invention, the present invention provides methods of forming a vibrating beam transducer having electrodes that exhibit the above mentioned desired properties.